(1) Field of the Invention
This invention relates to a cadmium non-sintered negative electrode for use in an alkaline storage cell like a nickel-cadmium storage cell and its manufacturing method.
(2) Description of the Prior Art
As a cadmium negative electrode for use in an alkaline storage cell like a nickel-cadmium storage cell, a paste-type cadmium non-sintered negative electrode is widely used because it is manufactured easily and at a low cost. This type of cadmium negative electrode is produced by kneading a cadmium active material like powdered cadmium oxide or powdered cadmium hydroxide with a liquid binder to obtain a paste, coating the paste on a conductive substrate and drying it. However, this type of negative electrode has a problem that a cell employing it has a shorter cycle life than a cell employing a cadmium sintered negative electrode. This is attributed to the following two points (a) and (b):
(a) Capacity deterioration of the cadmium negative electrode.
It is known that a cadmium non-sintered negative electrode produced in the above method deteriorates its capacity in the course of charge-discharge cycles.
To solve this problem, U.S. Pat. No. 3,870,562 has proposed manufacturing a cadmium non-sintered negative electrode by mixing cadmium oxide, metal cadmium comprising globe grains having an average grain size of 3 to 12 xcexcm, and 0.1 to 5% of nickel hydroxide. The above patent says the above manufacturing method can restrict capacity deterioration which is caused by inactivation of the cadmium active material accompanied by charge-discharge cycles.
However, a residual capacity ratio of a cell which has been left for a while, namely storage characteristic and cycle characteristic of a cell vary depending on the grain shape and grain size of added nickel hydroxide.
(b) Leak of electrolyte caused by pressure valve operation
The paste including the above active material is soft. Therefore, when the negative electrode plate coated with the paste is wound together with a positive electrode and a separator by a pressure roller or the like to make an electrode assembly, the outermost end of the negative electrode of the assembly is given more pressure than its innermost end, whereby the porosity of the negative electrode plate is reduced. If the negative electrode plate has not been subjected to electrochemical formation when employed in a cell, the negative electrode plate retains a smaller amount of electrolyte, compared with if the same negative electrolyte plate has been subjected to electrochemical formation when employed in a cell. Therefore, in the former case, undischarged cadmium is accumulated in the cadmium negative electrode in the course of charge-discharge cycles. Accordingly, the porosity of the negative electrode is increased, whereby the amount of electrolyte retained in the negative electrode is increased. As a result, the amount of electrolyte retained in the separator is reduced, and so the cycle characteristic is lowered.
To solve this problem, another method has been proposed in which the above paste is coated on a conductive substrate to obtain an electrode plate and this plate is prehydrated in an aqueous solution of alkali. According to this method of chemically prehydrating cadmium oxide, cadmium hydroxide formed of large crystals is obtained. Therefore, comparatively large holes are uniformly formed in the paste-type active material and also the mechanical strength of the negative electrode plate is increased. Accordingly, even if the negative electrode plate is wound around by a pressure roller to make an electrode assembly, its porosity is kept uniformly high. If sodium hydroxide is used as an aqueous solution of alkali for prehydration, the obtained cadmium hydroxide is formed of xcex3-type crystals, which make the cell easy to charge. This means that generation of hydrogen gas from the negative electrode plate is restricted, and so the inner pressure of the cell is prevented from increasing.
Though prehydration in an aqueous solution of sodium hydroxide takes time, it can be shortened by raising the temperature of the solution of sodium hydroxide.
However, prehydration is conducted in too a high temperature results in obtaining cadmium hydroxide formed by xcex2-type crystals instead of xcex3-type ones. Since xcex2-type crystals are large enough to make the cell hard to charge, hydrogen gas is easily generated and so the inner pressure of the cell is increased. As a result, the pressure valve is operated to leak the electrolyte and then deteriorate the cell cycle characteristic.
Accordingly, this invention has a primary object of offering a cadmium non-sintered negative electrode for increasing cell cycle characteristic and its manufacturing method.
Another object of this invention is to offer a cadmium non-sintered negative electrode for increasing cell storage characteristic and its manufacturing method.
Still another object of this invention is to offer a cadmium non-sintered negative electrode for restricting hydrogen gas generation and its manufacturing method.
Still another object of this invention is to offer a cadmium non-sintered negative electrode which can be manufactured in simple procedures and its manufacturing method.
The above objects are fulfilled by a cadmium non-sintered negative electrode for use in an alkaline storage cell, comprising a main active material comprising at least one of powered cadmium oxide and powdered cadmium hydroxide; a reserve charging substance comprising powdered metal cadmium; and an additive comprising powdered nickel hydroxide whose grain shape is substantially globe.
The above objects are also fulfilled by a method of manufacturing a cadmium non-sintered negative electrode for use in an alkaline storage cell, comprising the steps of kneading a main active material comprising at least one powdered cadmium oxide and powdered cadmium hydroxide, a reserve charging substance comprising powdered metal cadmium, an additive comprising powdered nickel hydroxide whose grain shape is substantially globe, a binder and a dispersion medium to obtain an active material paste; coating a conductive substrate with the active material paste; and drying the active material paste.
The nickel hydroxide may be added in a weight ratio (%) of 0.5 or more and 3.0 or less.
The powdered metal cadmium may be formed of fine grains.
The powdered metal cadmium may have an average grain size of 1.0 to 2.4 xcexcm.
The powdered metal cadmium may be produced by substitute reaction of powdered metal zinc and a solution of cadmium sulfate. The above-obtained powdered metal cadmium desirably has a specific surface area of 1 to 2 m2/g, or more desirably 1 to 1.5 m2/g. Though metal cadmium obtained by the Atomizer method usually has a specific surface area of 0.1 m2/g or less, the specific surface area exceeding 0.1 m2/g is desirable.
The reasons will follow.
The nickel hydroxide which has conventionally been used is formed by various-shaped grains. (This type of nickel hydroxide will be referred to as various-shaped nickel hydroxide hereinafter.) Being chemically unstable, the nickel hydroxide formed of various-shaped grains throws out foreign substances such as NO3xe2x80x94, or SO42xe2x80x94 included in its own crystals in small amounts into the active material during the cell charge-discharge in the alkaline electrolyte. As a result, the storage characteristic of the cell is deteriorated. On the other hand, the nickel hydroxide formed of globe grains (will be referred to as globe nickel hydroxide) used in the present invention are extremely chemically stable (confirmed through experiments). Therefore, it hardly throws out the above foreign substances even during charge-discharge. In consequence, an alkaline cell employing the globe nickel hydroxide restricts the decline of its storage characteristic and its cycle characteristic is excellent.
The above objects are also fulfilled by a method of manufacturing a cadmium non-sintered negative electrode for use in an alkaline storage cell, comprising the steps of kneading a main active material comprising at least one of powdered cadmium oxide and powdered cadmium hydroxide, a reserve charging substance comprising powdered metal cadmium, an additive comprising powdered nickel hydroxide, a binder and a dispersion medium to obtain active material paste; coating a conductive substrate with the active material paste; drying the active material paste to obtain an unhydrated electrode plate; and prehydrating the unhydrated electrode plate in a solution of sodium hydroxide.
The binder may comprise at least one of the group consisting of methyl cellulose, polyvinyl alcohol and carboxymethyl cellulose.
The dispersion medium may comprise a water including at least one oxyacid salt selected from the group consisting of phosphate, silicate, arsenate and chromate.
The active material paste may further include a reinforcing agent.
The reinforcing agent may comprise at least one of the group consisting of polyamide staple, polyolefine staple and acrylic staple.
One of the reasons of the problem of the prior art, namely capacity deterioration of the cadmium negative electrode, is solved by the above construction and method.
The above deterioration is caused by inactivation of the cadmium active material, which is attributed to the following two phenomena: (a) in the course of charge-discharge cycles, cadmium as a main active material is dissolved and deposited in repetition although by a very small amount, whereby its grains get large; and (b) surfaces of the grains of the cadmium active material are covered with cadmium hydroxide which has been formed by discharging, whereby the cadmium active material remains undischarged and is not involved in charge-discharge reaction.
Since the cadmium negative electrode lowers its capacity in the course of charge-discharge cycles, the cell capacity gets determined by the negative electrode capacity. However, if nickel hydroxide is added to the cadmium active material as in the present invention, the above two phenomena can be restricted. It has been confirmed through experiments that if the added nickel hydroxide has an average grain size of 1.5 xcexcm or more, its effect can be especially conspicuous. The reason is: nickel hydroxide grains of that size or nickel ions existing between the grains of the cadmium active material attract the cadmium, thereby to reduce the amount of the cadmium dissolved in the electrolyte, and also to restrict the grains of the cadmium active material from getting large.
It should be noted that the average grain size of more than 200 m is not desirable because nickel hydroxide formed of so large grains is hard to add uniformly to the cadmium negative electrode.
Therefore, the desirable average grain size of the nickel hydroxide is 1.5 to 200 xcexcm.
The above objects are also fulfilled by a cadmium non-sintered negative electrode for use in an alkaline storage cell, comprising a main active material comprising at least one of powdered cadmium oxide and powdered cadmium hydroxide; a reserve charging substance comprising powdered metal cadmium; and an additive comprising powdered nickel hydroxide whose average grain size is 1.5 to 200 xcexcm.
The above objects are also fulfilled by a method of manufacturing a cadmium non-sintered negative electrode for use in an alkaline storage cell, comprising the steps of kneading a main active material comprising at least one of powdered cadmium oxide and powdered cadmium hydroxide, a reserve charging substance comprising powdered metal cadmium, an additive comprising powdered nickel hydroxide having an average grain size of 1.5 to 20 xcexcm, a binder and a dispersion medium to obtain an active material paste; coating a conductive substrate with the active material paste and drying the active material paste.
The powdered metal cadmium may be formed of fine grains.
The other reason of the problem of the prior art, namely leak of electrolyte caused by pressure valve operation, is solved by the above construction and method.
It has been confirmed through experiments that, when cadmium oxide is hydrated in an aqueous solution of sodium hydroxide, the reacting speed is increased by raising the temperature of the sodium hydroxide solution. For example, the reaction takes an hour by the solution of 20xc2x0 C. while it takes only 10 minutes by the solution of 70xc2x0 C. When magnesium oxide is used as a dendrite preventor as disclosed in Japanese Patent Publication laid-open No. 56-30259 and Japanese Patent Publication No. 62-15994, cadmium hydroxide formed of xcex3-type crystals which make the cell easy to charge is obtained if the solution temperature is 20xc2x0 C. However, if the prehydration is conducted quickly in the solution of as high as 70xc2x0 C., the obtained cadmium hydroxide is mainly formed of xcex2-type crystals (FIG. 1). Since xcex2-type crystals are large, the surface area of the cadmium hydroxide is decreased and hydrogen overvoltage is lowered. Especially when the cell is charged at a low temperature, hydrogen gas is easily generated.
However, it has been confirmed that if nickel hydroxide is added as a dendrite preventor as in the present invention, cadmium hydroxide formed of xcex3-type crystals is obtained even if the prehydration is conducted at a high temperature (FIG. 2). Since xcex3-type crystals are very fine, the surface area of the cadmium hydroxide is increased. Therefore, the obtained cell is easy to charge and can restrict hydrogen gas generation. Since the inner pressure of the cell is not raised, there is no operation of the pressure valve or electrolyte leak. As a result, the cycle characteristic can be kept high.